One model of leukemogenesis involves the generation of dual-tropic murine leukemia viruses (MuLVs) by molecular recombination between the spontaneously induced ecotropic MuLV and endogenous proviral DNA segments present in the chromosomal DNA of the mouse. Following the recombination event, a dual-tropic MuLV provirus may gain entry into a susceptible cell (such as a lymphocyte in the thymus) and, if integration occurs at an appropriate site (near a putative oncogene), disease may occur. Biological and genetic studies of inbred mouse strains indicate that endogenous ecotropic proviruses are differentially expressed in various strains of mice. AKR mice, which may harbor as many as five copies of such endogenous ecotropic proviral DNAs, begin producing virus shortly after birth, and 100% of animals develop leukemia within the first year of life. In contrast, BALB/c animals have only a single endogenous ecotropic provirus, sporadically produce low titers of ecotropic MuLVs, and only a minority (10 to 15%) develop disease. To evaluate why BALB/c animals inefficiently express ecotropic proviruses, we decided to molecularly clone the single copy of viral DNA present and compare it to the ecotropic proviruses found in AKR animals. Differences in the expression of infectious MuLVs could be due to alterations in viral structural or regulatory genes or might reflect the influence of flanking cellular DNA sequences. During the past year, we utilized cosmid and Lambda phage vectors to isolate the ecotropic provirus from BALB/c chromosomal DNA. Five clones were isolated from a BALB/c library subsequent to the screening of 1.5 million plaques. All five clones contained the 3 feet LTR and from 3.5 to 5 kb of pol and env retroviral sequences. Restriction mapping and blot hybridization tehniques have verified an ecotropic proviral DNA has been cloned. The cloned segment is being transferred to a suitable plasmid vector prior to its assay in biological systems. The nucleotide sequence of the 3 feet LTR will be determined and compared with the previously published sequence for the highly infectious and readily inducible ecotropic proviral DNA isolated from AKR mice.